Chemistry Reference
In-Depth Information
When the polymers are doped with a hole-transporting triphenylamine, an enhancement in photo-
conductivity in the visible region is observed. This suggests that the metal complexes serve as
photosensitizers instead of charge carriers [
260
]. Chan et al. [
260
] observed an electric field-dependent
charge separation process in these polymers. It is described well by the Onsager's theory of charge
germinate recombination. This theory assumes that some fraction of absorbed photons produce bound
thermalized electron-hole pairs that either recombine or dissociate under the combined effects of the
Coulombic attraction and the electric field. The photo-generation efficiency is given as the product of
the quantum yield of thermalized pair formation and the pair dissociation probability:
"
#
1
1
g¼
0
I
g
2
eEr
0
kT
e
eEr
0
kT
Fðr
0
; EÞ¼F
0
1
I
g
4
pe
0
e
r
kTr
0
where
I
g
is a recursive formula given by
I
gþ
1
ðxÞ¼I
g
ðxÞx
gþ
1
exp
ðxÞ=ðg þ
1
Þ
where
I
0
ðxÞ¼
1
exp
ðxÞ
,
F
is the primary yield of thermalized bound pairs,
r
0
is the initial
thermalization separation between the bound charges,
e
r
is the relative permeability, and
E
is the
applied electric field strength.
Jenekhe and de Paor [
261
] reported exciplex formation and photoelectron transfer between several
n-type
(
electron accepting
)
-conjugated rigid-rod polymers and donor triarylamine molecules. In
particular, they reported an investigation of an
n-type
conjugated polymer poly(benzimido azobenzo-
phenanthroline ladder) [
132
]. No evidence was observed by them of a ground state charge transfer or
any strong interactions between the conjugated polymer pairs. Transient absorption spectra of a blend
of thin films in the 420-730 nm region were obtained at various time delays following photo-
excitation at 532 nm. Dramatically enhanced photo-induced bleaching in the 430-480 nm region
was observed. Jenekhe and de Paor propose that they observed enhanced photo-bleaching in the
blends and that it is a consequence of photo-induced electron transfer [
261
]. The electron transfer was
illustrated as follows:
p
N
S
S
N
n
UV light
O
O
N
N
N
N
n
N
S
n
S
N
+
N
N
N
N
n
O
O
